A typical wood-frame structure rests on a concrete foundation. The bottom framing element of each wall of the structure, which directly contacts the foundation, is called the sill. The sill can be attached to the foundation to prevent the structure from sliding off the foundation. Devices for attaching the sill to the foundation are referred to as hold-downs.
Presently, traditional bolt and nut systems are used to hold down the wooden sill of a wood-framed structure. First, a bolt is sunk into the foundation. Next, workers drill a hole in the sill for each bolt, and the sill is placed on the foundation so that the bolts pass through the holes. Finally, a nut is mated with each bolt and tightened securely against the sill. The downward pressure exerted on the sill by the nut holds the sill, and thereby the structure, in place. Consequently, the area of the sill directly underneath the nut is critical to the effectiveness of this hold-down system.
However, the wooden sill tends to deteriorate with age, for several reasons. The structure above the sill exerts a substantial weight on it. Over time, that weight alone can compress or crush the wooden sill. As the wood dries out, it can shrink; further, it may become more brittle, providing less resistance against the constant weight of the structure above. Termites or other insects may attack the sill, destroying its integrity and rendering it vulnerable to crushing. Hurricanes, tornados, and earthquakes can create severe shear stresses and uplifting forces in the walls of the structure, repeatedly forcing the sill against the nut and weakening, crushing, or splintering the wood directly underneath the nut. Overtightening the nut at the time of construction can crush or splinter the wood underneath the nut, precisely where the nut and bolt system requires a snug fit in order to function.
As the sill deteriorates, and as the wooden area underneath the nut deteriorates, the position of the nut remains constant relative to the bolt. Consequently, a gap forms between the nut and the sill. This gap significantly weakens the hold-down mechanism. It allows the sill, and the structure, to shift relative to the foundation. The resultant fatigue loads cause the sill to deteriorate further underneath the nut, widening the gap and rendering the hold-down progressively weaker. When strong forces such as those produced by a hurricane, tornado, or earthquake create severe shear and uplift forces in the wood-framed structure, the gap between the nut and the sill destroys the functionality of the hold-down by allowing those shear and uplift forces to shake the structure up and down, increasing the likelihood of its damage or destruction.